It would be interesting to know the comparison of noise performance of 5DIII and 5DII. Is it worth to do investment in upgrading 5DII to 5DIII? There would be no answer, but more information for you to make your own decision.
As my colleague bought a new 5DIII, I performed a side-by-side comparison of dark frame under a control condition. Both cameras are unmodified. The experimental conditions are following:
ISO: 400, 800, 1600,3200, 6400, 12800, 25600, 51200 (5DIII only), 102400 (5DIII only)
Noise reduction is turned off
Exposure time: 5 mins
Room Temp: 24oC
For the 5DII, basically the noise is low up to ISO 1600. One can see there is only a narrow peak at the left hand side of the histogram. However, when the ISO is push to 3200 and beyond, there is significant increase in noise. The histograms are wide spread distribution.
For the 5DIII, the trend of the noise performance is similar to that of 5DII. But the limit is further pushed. The noise is very low up to ISO 3200. In fact, if you compare the 5DII's ISO 1600 histogram with 5DIII's ISO 3200 histogram, the later one got less area! That is the 5DIII has lower noise. However, if we look at the 5DIII's ISO 6400 histogram, it couldn't beat 5DII's ISO 1600. Since I did the test in 1-stop ISO increment only, I guess the 5DIII should beat 5DII by 1 1/3-stop to 1 2/3-stop.
Here I don't mean the usable ISO of 5DII is 1600 for long exposure. Whether the noise is acceptable is a personal matter. I just pointed out that the noise would increase significantly under this experimental condition. In addition, If one is satisfied with the noise level of ISO 1600 of 5DII, one should also satisfied with ISO 4000-5000 of 5DIII.